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epoxy-functionalized polydimethylsiloxane reinforced with POSS
Composites Part B 75 (2015) 226e234
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Novel nanocomposites based on epoxy resin/epoxy-functionalized polydimethylsiloxane reinforced with POSS N.M. Florea a, b, A. Lungu a, P. Badica c, L. Craciun b, M. Enculescu c, D.G. Ghita b, C. Ionescu b, R.G. Zgirian a, H. Iovu a, d, * a
University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu Street, 011061, Bucharest, Romania Horia Hulubei e National Institute for Physics and Nuclear Engineering (IFIN-HH), 30 Reactorului Street, 077125, Magurele, Romania National Institute of Materials Physics, Atomistilor 105 bis, 077125, Magurele, Romania d Academy of Romanian Scientists, 54 Splaiul Independentei, 050094, Bucharest, Romania b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2014 Received in revised form 23 December 2014 Accepted 26 January 2015 Available online 4 February 2015
The purpose of the present study is to develop novel nanocomposites based on diglycidylether of bisphenol A (DGEBA) combined with diglycidylether-terminated polydimethylsiloxane (DG-PDMS), reinforced with 10 wt.% (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS). DG-PDMS and POSS compounds were covalently incorporated into DGEBA resin via copolymerization of epoxy groups. The effect of both DG-PDMS and POSS nanoparticles on the curing reaction, glass transition temperature (Tg), thermal stability, hardness and morphology of DGEBA/DG-PDMS ± POSS nanocomposites were studied by DSC, FTIR, DMA, TGA, SEM/EDX, AFM and contact angle measurements. SEM/EDX and AFM results prove that OEP-POSS is well dispersed within DGEBA/DG-PDMS polymer matrix, while MEP-POSS forms large POSS aggregates. The thermo-mechanical properties of POSS based nanocomposites are also in good correlation with morphology features. MEP-POSS based nanocomposite with heterogeneous dispersion of POSS aggregates exhibits lower Tg value and thermal stability in comparison with OEP-POSS nanocomposite which exhibits a nanoscale dispersion of the POSS cages. The obtained Tg of OEP-POSS based nanocomposite increases with 31 C in comparison with the unreinforced matrix. Moreover, this nanocomposite shows the highest storage modulus (E0 ) and hardness. © 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer-matrix composites (PMCs) A. Nano-structures B. Thermomechanical D. Electron microscopy Polyhedral oligomeric silsesquioxanes (POSS)
1. Introduction Over the past decade, advanced researches have been focused on the development of polymer based nanocomposites, reinforced with well-defined nanostructured compounds, which are expected to provide high performance materials, with potential application in a wide range of technological areas [1]. Recently, polyhedral oligomeric silsesquioxanes (POSS) nanostructures have gained considerable attention as they possess a synergistic combination of constituent properties of conventional inorganic materials and organic polymers, resulting in a new generation of hybrid materials with superior properties [2e5]. POSS molecules architecture is based on a well-defined polyhedral cage
* Corresponding author. University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu Street, 011061, Bucharest, Romania. Tel.: þ40 214023922; fax: þ40 213111796. E-mail address: [email protected] (H. Iovu). http://dx.doi.org/10.1016/j.compositesb.2015.01.043 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
(1.5 nm), with general formula (RSiO3/2)n, n varying from 6 to 18. These macromers have an inorganic silica-like core with a precise geometry, surrounded by organic ligands (R) covalently bonded to each Si atoms placed at the vertices of the polyhedral cage. R may be hydrogen or any alkyl, alkylene, aryl, arylene groups, or organofunctional derivatives. Unlike silica, silicones and other conventional inorganic fillers, the POSS compounds exhibit advantages like monodisperse size, low density, and synthetically wellcontrolled functionalities. The functional groups from the organic substituents bring chemical versatility to POSS nanoparticles and make POSS widely compatible with various organic polymers [6e8]. Epoxy resins, the well-known class of thermosetting polymers, have been widely used as adhesives, electronic encapsulating compounds, and organic phase of composite materials due to their high mechanical strength, high chemical and corrosion resistance, excellent electrical insulation and simplicity in processing [9,10]. For further improvements of the general properties of epoxy resins,
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different POSS compounds containing epoxy, hydroxyl and amino reactive functionalities which can generate covalent bonding with the base polymer to reduce the agglomeration effect, have been employed. In general, the obtained results showed that the incorporation of POSS nanounits within epoxy resins improves the performances of the obtained networks [11e13]. However, even for POSS containing reactive groups, small POSS aggregates can form within the base polymer that lead to lower thermo-mechanical properties [14]. Moreover their highly cross-linked structures exhibit a brittle behavior, poor crack resistance, and low fracture toughness. One of the most promising approaches used to modify the features of such brittle polymers consists in adding of flexible polymers like polysiloxanes, used as spacers between POSS and polymer matrix during the preparation of POSS/polymer nanocomposites. This method has been demonstrated to be efficient in order to prepare nanocomposites with good dispersion and enhanced thermo-mechanical properties [15e19]. Polysiloxanes have been extensively used to improve the epoxy resins properties due to their excellent thermal stability [20]. Moreover, these compounds were also used in the preparation of different POSS/ polysiloxane composites. Chen and coworkers demonstrated that the incorporation of POSS nanoparticles into polysiloxane elastomers can improve the thermal stability of the obtained composites [21]. Recently, we have reported the synthesis and characterization of simultaneous interpenetrating polymer networks (IPN) based on dimethacrylic/epoxy resins via in situ polymerization reinforced with octafunctional POSS (with methacrylate or epoxy groups) [22]. In this study, we successfully proved that the degree of POSS dispersion within the polymer matrix is a key factor that is strongly influenced by the organic substituents grafted onto the POSS cage. The obtained results show that the incorporation of OEP-POSS within the used epoxy resin lead to a decrease of thermomechanical properties (Tg and thermal stability) of the obtained nanocomposites. Thus, based on our previous experience [23e31] we intended to design different organic-inorganic hybrid materials with good dispersion and enhanced thermo-mechanical properties by combining diglycidyl ether of bisphenol A (DGEBA) with a small quantity (2 wt.%) of diglycidyl ether terminated poly(dimethylsiloxane), (DG-PDMS). As reinforcing agents, (mono-/octa) epoxy POSS (POSS-(3-Glycidyl) propoxy-Heptaisobutyl substituted (MEP-POSS) and POSS-Octa-(3-glycidyloxypropyl) dimethylsiloxy) (OEP-POSS)) were included into DGEBA/DG-PDMS organic host. To improve the dispersion of POSS during the preparation of POSS/ polymer nanocomposites, DG-PDMS was used as a spacer between POSS molecules and DGEBA, enabling a high mobility of the epoxy chains which may contribute to a better dispersion of POSS units within the polymer matrix. To the best of our knowledge, all previously studies were focused mainly on synthesis of different POSS/epoxy resin and POSS/PDMS nanocomposites, while DGEBA/DG-PDMS nanocomposites reinforced with epoxy POSS compounds remain unexplored [12e16,32e35]. Thus, we focused our research to evaluate the effect of DG-PDMS and epoxy POSS nanostructures on the properties of epoxy network. The curing behavior of DGEBA/DGPDMS reinforced with epoxy POSS was analyzed by DSC and FTIR spectroscopy. The thermal stability and morphology of the obtained nanocomposites are evaluated by DMA, TGA, SEM/EDX, AFM and contact angle measurements. The hardness of the studied materials was also investigated through the AFM nanoindentation tests.
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2. Experimental section 2.1. Materials The studied networks possess in their structures a mixture of diglycidyl ether of bisphenol A (DGEBA), D.E.R.™ 332 (epoxy equivalent weight of 171e175 g/eq) and diglycidyl ether terminated poly(dimethylsiloxane), (DG-PDMS) (epoxy equivalent weight of 490 g/eq and a Mn ~ 800 g/mol). Two types of epoxy POSS (POSS-(3Glycidyl)propoxy-Heptaisobutyl substituted with Mw ¼ 931.63 g/ mol (MEP-POSS) and POSS-Octa(3-glycidyloxypropyl)dimethylsiloxy) with Mw ¼ 931.11 g/mol (OEP-POSS)) were used as reinforcing agents. 1 methyl imidazole (1MeI) was the anionic initiator for the high temperature polymerization reaction of epoxy groups. All reagents, were supplied by Sigma-Aldrich Chemicals, and used without further purification. The chemical structures of the raw materials are further illustrated in Scheme 1. 2.2. Formation of POSS based DGEBA/DG-PDMS nanocomposites As represented in Scheme 1, two types of POSS based DGEBA/ DG-PDMS nanocomposites were synthesized. Firstly, DGEBA/DGPDMS polymer matrix was prepared by mixing the liquid epoxy resin (DGEBA) with the epoxy functionalized polydimethylsiloxane (DG-PDMS) in 98/2 weight ratio. Further, 10 wt.% MEP or OEP POSS was incorporated within DGEBA/DG-PDMS organic host. To achieve a good dispersion of both DG-PDMS and POSS nanocages within DGEBA resin, the obtained mixture was sonicated 1 h at room temperature using an ultrasonic processor UP100H. After mixing the components, 5 wt.% 1MeI was added. The resulting blends were degassed to remove the entrapped air during mixing, poured into Teflon molds, cured at 120 C for 2 h, and postcured at 160 C for another 3 h. After cooling, the obtained hybrids were removed from the mold and characterized. DGEBA homopolymer was also prepared in the same conditions. In order to check the efficiency of the network formation and to remove any traces of unreacted compounds, the synthesized samples were Soxhlet extracted with a suitable solvent for 2 h and subsequently were dried at room temperature for minimum 24 h. After extraction, no shape deformation was observed for any specimen. 2.3. Characterization Differential scanning calorimetry (DSC). The calorimetric measurements were performed on a Netzsch DSC 204 F1 Phoenix differential scanning calorimeter under a constant nitrogen flow rate (20 mL/min). All the samples (about 4 mg in weight) were nonisothermally heated from 20 to 350 C and the DSC curves were recorded at a heating rate of 10 C/min. Fourier transform infrared (FTIR) spectra were registered on a Vertex 70 Bruker FTIR spectrometer equipped with an attenuated total reflectance (ATR) accessory in order to determine the chemical structure and also the curing degree of all the studied specimens. All FTIR measurements were performed in the ATR-FTIR cell on Ge crystal, at room temperature. The FTIR spectra were recorded using 32 scans in 600e4000 cm1 wavenumber region. The dynamic mechanical behavior of the samples after postcuring was followed on a Tritec 2000 (Triton Technology) instrument using 10 mm 2 mm 20 mm rectangular specimens in a single cantilever bending mode. Tan d was calculated as a function of the temperature ranging from 130 to 200 C using 5 C/min heating
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Scheme 1. The synthesis route of DGEBA/DG-PDMS ± POSS nanocomposites.
rate at a frequency of 1 Hz. The displacement amplitude was 0.05 mm. The experiment was performed in triplicate. The thermogravimetric analysis (TGA) curves were achieved on a Q500 TA instrument. A sample of about 2.5 mg was placed in a platinum crucible and heated from 20 to 650 C at a heating rate of 10 C/min under a constant nitrogen flow rate (balance flow 10 mL/ min, oven flow 90 mL/min). Three different experiments were run for each sample. Scanning electron microscopy (SEM) and Si mapping of DGEBA/ DG-PDMS ± POSS hybrids were carried out using a Zeiss EVO50 scanning electron microscope equipped with a Carl Zeiss NTS energy-dispersive X-ray detector (EDX). Before analysis, cryogenic fracture surfaces were covered with a thin gold layer. Atomic force microscopy (AFM). Surface topography was examined by using MultiMode NanoScope IIID Controller Atomic Force Microscope (Digital Instruments Veeco Metrology Group, Santa Barbara, CA, USA) working in the so-called tapping mode. Images were achieved using a TESP (0.01e0.025 Ohm-cm Antimony (n) doped Si) tip at 1 Hz scan rate. All the measurements were performed in ambient environment. Images were flattened using first order fitting to remove any background tilt. All images were achieved at 10 mm, 5 mm and 2.5 mm size scale. In order to characterize the roughness of these samples, a 5 5 mm2 surface area was analyzed. The roughness measurements were performed over the entire image. A minimum of five specimens were tested for each sample and the average values are reported. The hardness of all the studied specimens was also investigated by nanoindentation measurements. Nanoindentation tests were performed at room temperature using the MultiMode NanoScope IIID Controller
atomic force microscope. A DNISP diamond tip mounted on a stainless steel cantilever (spring constant, k ¼ 227 N/m) was used for both indentation test as well as for AFM imaging. Nanoindentation hardness was defined as the maximum indentation load (F) divided by the projected area (A). The projected area was determined from the tip shape of the indenter using AFM NanoScope V531r1 software and Heron formula [36e38]. The indentation load was set to 98 mN and kept constant for all measurements. A 2 2 array of indentation was made on each sample, in three different areas, using the same force and parameters. The hardness was obtained as an average of twelve measurements. Contact angle measurements. The surface wettability was determined using KSV Instruments's CAM 100 by measuring the contact angle on the surface of the films with small water droplets. The contact angles reported represent the mean of three tests. 3. Results and discussion 3.1. The influence of POSS type on the DGEBA/DG-PDMS curing behavior studied by DSC To investigate the effect of both DG-PDMS and POSS type on the reactivity of epoxy groups during copolymerization reactions of DGEBA/DG-PDMS ± POSS nanocomposites, non-isothermal DSC scans were performed. Fig. 1 shows the DSC curves before curing for DGEBA and hybrids based on DGEBA/DG-PDMS ± POSS (MEP or OEP-POSS). The curve 1 from Fig. 1 reveals that DGEBA homopolymer exhibits an exothermic peak at 142 C, assigned to the temperature at
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Fig. 2. FTIR spectra of DGEBA/DG-PDMS system at different polymerization times. Fig. 1. DSC curves for: (1) DGEBA; (2) DGEBA/DG-PDMS; (3) DGEBA/DG-PDMS/MEPPOSS; (4) DGEBA/DG-PDMS/OEP-POSS.
which the polymerization enthalpy of epoxy groups is maximum (Tmax). As one may notice from Fig. 1 (curve 2), the presence of DGPDMS exhibits no significant influence on the polymerization of epoxy groups from pure DGEBA. The introduction of (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS) within DGEBA/DGPDMS matrix, causes an increase of Tmax which depends mainly on the number of the epoxy groups belonging to the POSS cages. Thus, the inclusion of MEP-POSS within DGEBA/DG-PDMS matrix, leads to a slightly increase of Tmax from 143 C to 147 C (curve 3). This type of POSS bearing only one reactive group exhibits a high agglomeration tendency which may lead to a partial POSS cages agglomeration which hinders the attack of the initiator (1-MeI) to the epoxy groups. When OEP-POSS is used, the epoxy polymerization occurs at a higher temperature (157 C, Fig. 1 curve 4) probably due to the restricted mobility of the DGEBA chains in the vicinity of the POSS cages, these chains being less available for the polymerization. The reactivity of epoxy groups belonging to this type of POSS is much lower than for neat DGEBA. In the case of OEP-POSS/1MeI the exothermic peak assigned to the maximum polymerization enthalpy of the epoxy groups appears at 194 C, as we previously reported [22].
ðA =A Þ h ¼ 1 915 1608 t 100 ðA915 =A1608 Þ0
(1)
where:
h e conversion, %;
A915 ¼ 915 cm1 peak area; A1608 ¼ 1608 cm1 peak area; t ¼ the reaction time (min); 0 ¼ initial moment of the reaction (uncured mixture). The obtained conversion degrees for DGEBA/DG-PDMS ± POSS nanocomposites are shown in Fig. 3. From the conversion results, it may be observed that the conversion degree of epoxy groups is strongly influenced by the presence of both DG-PDMS and POSS compounds (Fig. 3). From Fig. 3 curve 2, one may noticed that the incorporation of a small amount of low molecular weight DG-PDMS into DGEBA epoxy resin leads to lower conversions than for neat DGEBA probably due to the chains of DG-PDMS which may partially hinder the attack of 1MeI molecules to the epoxy groups.
3.2. FTIR studies during the polymerization of POSS based DGEBA/ DG-PDMS nanocomposites FTIR studies were used to monitor the curing reactions of the epoxy groups. The FTIR spectra shown in Fig. 2, were performed during the curing process of DGEBA/DG-PDMS hybrid and reveal that the peak assigned to the epoxy groups (915 cm1) significantly decreases at higher reaction times. After the postcuring treatment the corresponding absorption band disappeared, indicating that the curing reaction of epoxy groups is complete. Under the same curing and postcuring conditions similar results were obtained for POSS-containing nanocomposites. The characteristic peak in the infrared region for the epoxy ring disappeared, indicating that the curing reactions in the nanocomposites were carried out to completion. The band at 1608 cm1, assigned to aromatic C]C stretching vibration was used as a reference. The conversion degree of epoxy groups was calculated by means of Equation (1) from the ratio of the epoxy groups absorbance peak area (915 cm1) relative to that of the aromatic C]C stretching vibration (1608 cm1), for the cured and uncured specimens.
Fig. 3. The conversion degree of the epoxy groups against time for: (1) DGEBA; (2) DGEBA/DG-PDMS; (3) DGEBA/DG-PDMS/MEP-POSS; (4) DGEBA/DG-PDMS/OEP-POSS.
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The inclusion of POSS compounds, bearing one or eight epoxy moieties, decreases the rate of epoxy polymerization especially at the early stages, which is probably a consequence of the hindrance effect due to the bulky POSS cages [22e24,39]. Moreover, a significant influence of the number of epoxy groups from the POSS cages against conversion was noticed. Thus, in the early stages of the reaction this influence is not noticeable since the network formation is just at the beginning. However, when the reaction goes on, the crosslinking density becomes higher since more epoxy groups are involved. As a consequence, OEP-POSS based nanocomposites exhibit a high crosslinking density of the final network due to the high content of epoxy groups from the POSS molecules. In this case the access of the 1MeI molecules to the epoxy groups becomes more difficult which leads to a lower conversion of systems with OEP-POSS in comparison with those with MEP-POSS in which the formed network exhibit a lower crosslinking density. This behavior is probably caused by the agglomeration tendency of MEP-POSS. More details about morphology will be further described in morphology section. To achieve a complete cure for all the studied specimens, a postcuring treatment at a higher temperature was also performed. At the end of the postcuring process, all the systems were almost completely cured (Fig. 3). 3.3. Thermo-mechanical properties To establish the influence of DG-PDMS and POSS on the Tg and storage modulus (E0 ) of the cured DGEBA/DG-PDMS ± POSS networks, DMA tests were performed in the temperature range 130 ÷ 200 C (Fig. 4, Table 1). From the tan d curves it may be observed that the neat epoxy network (Fig. 4(a) curve 1) shows two transitions: one at 137 C assigned to the a-relaxation and corresponds to the glass transition of the epoxy network. The transition at low temperature (130 C to 20 C) is attributed to the b-relaxation of the epoxy chain, and is due to the motion of the glycerol groups in DGEBA [40,41]. The incorporation of 2 wt.% of low molecular weight DG-PDMS within DGEBA leads to a lower Tg of the cured DGEBA/DG-PDMS network (127 C) in comparison with DGEBA homopolymer. This behavior is caused mainly by the high flexibility of DG-PDMS chain. It is also clearly seen that DGEBA/DG-PDMS hybrid displayed a single well-defined Tg, which indicates that the epoxy
Fig. 4. The dependences of tan d (a) and storage modulus E0 (b) curves against temperature for DGEBA/DG-PDMS ± POSS nanocomposites.
Table 1 Thermomechanical results for DGEBA/DG-PDMS ± POSS hybrid materials. DGEBA/DG-PDMS ± POSS
DMA
TGA
Tga, ( C) Td5%b, ( C) Td50%c, ( C) Weight lossd, (%) DGEBA DGEBA/DG-PDMS DGEBA/DG-PDMS/MEP-POSS DGEBA/DG-PDMS/OEP-POSS a b c d
137 127 132 158
383 400 318 335
438 440 440 445
87 88 83 80
Tg ¼ glass transition temperature (maximum of tan d); Td5% loss ¼ the temperature at which the weight loss is 5%. Td50% loss ¼ the temperature at which the weight loss is 50%. The percentage of the weight loss at 600 C.
terminated PDMS form a homogeneous phase with the epoxy resin [16,42,43]. The dynamic mechanical behavior of the obtained PDMS/epoxy hybrids was determined using the storage modulus (E0 ) results. From Fig. 4(b) curve 2 it may be observed that the inclusion of the flexible DG-PDMS chains within DGEBA resin, leads to a lower E0 of the obtained hybrid network, in comparison with E0 of DGEBA neat matrix. For these types of hybrid networks a lower crosslinking density was achieved which lead to a lower E0 in the rubbery region [31]. The incorporation of MEP-POSS within the polymer matrix, leads to a slightly increase of Tg of the obtained nanocomposites in comparison with the unreinforced DGEBA/DG-PDMS organic host. This type of POSS bearing only one reactive group exhibits a selfeassembly tendency which may lead to a partial POSS cages agglomeration which subsequently leads to a lower crosslinking density. When OEP-POSS is used, the obtained value for the Tg of the corresponding nanocomposite is shifted with 31 C towards higher temperature compared with the polymer matrix. The effect of raising the Tg by the introduction of POSS cages into the organic host suggests that the polymeric matrix can be reinforced on the nanoscale level by the POSS cages which are covalently bonded to the polymer networks [8,44]. OEP-POSS seems to be the best candidate to form a highly crosslinked network due to the presence of multiple epoxy groups. The rigid and bulky structure of POSS cages could restrict the motions of macromolecular chains which additionally enhance the Tg of the OEP-POSS based nanocomposites. Moreover, for OEP-POSS based nanocomposites, it was noted that the tan d curve was significantly decreased in height and becomes broader. This suggests higher limitations on freedom of chain mobility in the samples probably due to an increased crosslinking density [31,43]. Concerning the dynamic mechanical properties of the POSS reinforced systems, it was interesting to note that in the glass state region the storage modulus of MEP-POSS reinforced nanocomposites is lower than DGEBA/DG-PDMS. This behavior is probably caused by the heterogeneous structure of the obtained nanocomposites in which the POSS compounds form large aggregates. Moreover, the presence of bulky POSS aggregates leads to an increase in the free volume between the polymer chains thus acting like a plasticizer. In the rubbery domain, the dynamic storage modulus E0 appears to be improved, no matter what type of POSS was used. The highest storage modulus was achieved for the nanocomposites containing OEP-POSS which can easily react with epoxy groups from both DGEBA and DG-PDMS resulting nanocomposites with higher crosslinking density. These networks exhibit a higher stiffness even in comparison with DGEBA pure matrix.
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Fig. 5. TGA plots for: (1) DGEBA; (2) DGEBA/DG-PDMS; (3) DGEBA/DG-PDMS/MEPPOSS; (4) DGEBA/DG-PDMS/OEP-POSS.
3.4. Thermal stability The thermal stability of the DGEBA/DG-PDMS ± POSS hybrid materials was revealed through TGA measurements (Fig. 5). Within the experimental temperature range all the TGA curves are unimodal, displaying only one degradation step, suggesting that the incorporation of DG-PDMS and epoxy POSS exhibits no significantly influence on the thermodegradation mechanism of the epoxy polymer matrix. From the TGA curves, it can be calculated that the Td5% value obtained for DGEBA/DG-PDMS hybrid is 17 C higher compared with the Td5% registered for DGEBA homopolymer. This behavior clearly indicates that the incorporation of DG-PDMS within epoxy resin increases the thermostability of the obtained DGEBA/DGPDMS hybrid. The presence of SieOeSi inorganic backbone on
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the DG-PDMS structure contributes to the increase of the thermostability due to the high SieO bond dissociation energy (460 kJ/ mol) compared with the CeO (345 kJ/mol) or even with SieC (318 kJ/mol) bonds [45]. The nanocomposites reinforced with POSS (10 wt.%) were also subject to thermal analysis. The TGA results show that the MEPPOSS/OEP-POSS based nanocomposites exhibit lower initial thermal stability compared with the unreinforced network, DGEBA/DGPDMS. Among POSS based nanocomposites, the lowest temperature corresponding to the weight loss of 5% was obtained in the case of MEP-POSS nanocomposite which is probably due to the low crosslinking density of the network established between MEP-POSS and epoxy matrix. Although the initial thermal stability is significantly decreased, at higher temperatures, the degradation degree (Td50%) is almost similar for POSS based nanocomposites as for the unreinforced material. As Fu et al. previously reported [46], this behavior can be attributed to the yields of silicon-rich residues, which make the materials more stable at elevated temperatures. Therefore, the thermal degradation behavior of epoxy resins is strongly influenced by the presence of POSS compounds due to the self-decomposition and self-condensation processes which occur at relatively low temperatures and form protective layers that increase the thermostability at higher temperatures. It should be emphasized that the lowest degradation degree at higher temperatures was observed for the nanocomposite reinforced with OEP-POSS, among all the POSS based nanocomposites. This improvement could be the result of the formation of tightly crosslinked structure between the polymer matrix and POSS cages. In this case, the POSS cages are probably dispersed at nanoscale level in epoxy matrix. For the nanocomposite with MEP-POSS, the degradation rate at higher temperatures (Td50%) remains unchanged in comparison with unreinforced matrix. This type of POSS exhibits a strong tendency toward aggregation and thus its presence tends to reduce the epoxy crosslinking density. Therefore, the decomposition
Fig. 6. SEM and EDX Si mapping micrographs of the fractured surface of POSS based DGEBA/DG-PDMS nanocomposites.
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temperature of the epoxy resin will decrease. These results are in good agreement with DMA tests. As we expected, the char yield increases for the POSS based nanocomposites, because of the presence of the inorganic moiety.
3.5. Morphological characterization To establish the morphology of the DGEBA/DG-PDMS ± POSS hybrid materials, SEM with EDX Si map distribution and AFM analyses were performed and the obtained results are displayed in Figs. 6 and 7(aed), respectively.
The SEM and Si mapping images, show that DG-PDMS is homogenously distributed in the corresponding epoxy matrix. SEM and EDX Si mapping micrographs are also performed for the POSS based nanocomposites. The obtained results reveal that the OEPPOSS seems to be well dispersed in the network, and no aggregates were observed. This good dispersion of OEP-POSS hybrid nanobjects can be attributed to the miscibility of the POSS within the resin mass and to the covalent bonds formed between the eight epoxy functionalities belonging to POSS nanocages and the epoxy groups from the base matrix. Studying the morphology of MEP-POSS containing networks, it is obviously that this type of POSS exhibits a highly agglomeration
Fig. 7. The AFM micrographs of (a) DGEBA; (b) DGEBA/DG-PDMS; (c) DGEBA/DG-PDMS/MEP-POSS; (d) DGEBA/DG-PDMS/OEP-POSS.
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tendency that leads to a phase segregation in the matrix which is in good agreement with DMA results. The obtained results are also in good agreement with those reported by Bo cek et al. [47]. To fully understand the morphology of the POSS based materials, AFM analyses were performed on all the synthesized nanocomposites and the obtained 3D images are displayed in Fig. 7(aed). AFM is also a useful tool for studying the surface roughness at the nanoscale level. To characterize the roughness of the studied samples, 5 mm2 surface areas were analyzed and the obtained average roughness (Ra) values are illustrated in Fig. 8. Applying special investigation modes, AFM can additionally be used to measure mechanical properties such as nanoindentation hardness. The results of hardness measurements performed for the studied samples are illustrated also in Fig. 8 along with the roughness values. From the 3D AFM images shown in Fig. 7aeb and from the average roughness (Fig. 8), it may be observed that DGEBA/DGPDMS hybrid and DGEBA homopolymer exhibit similar surface topographies and similar roughness values indicating that DGPDMS is well dispersed within DGEBA epoxy matrix and no phase segregation occurs. For the POSS based nanocomposites different morphologies were obtained (Fig. 7ced). The incorporation of MEP-POSS into DGEBA/DG-PDMS matrix yields to a considerable increase in the surface roughness as shown in Fig. 7c and Fig. 8. MEP-POSS being functionalized with one epoxy group and seven unreactive isobutyl groups, exhibits a high tendency to self-agglomerate at the surface of the obtained nanocomposite. Our results for MEP-POSS are completely consistent with the results obtained by Dodiuk et al. [48] who describes that an increased roughness is accompanied with a reduced Tg and storage modulus. In this case, POSS nanocages act more like a plasticizer than as reinforcing agent. Consecutively, it was concluded, that a poor compatibility is obtained between this type of POSS and the polymer matrix. The nanocomposite with OEP-POSS exhibits a roughness value similar with the unreinforced samples, which means that this POSS is well dispersed in the polymer matrix due to the eight epoxy groups attached to SieOeSi cages. In this case a higher compatibility between the POSS molecules and the polymer matrix was achieved. Concerning the nanoindentation hardness features, it was noticed that the presence of 2 wt.% of the polysiloxane monomer generate minor modifications [16]. As expected, the addition of 10 wt.% POSS into neat DGEBA/DG-PDMS matrix has a great effect on the nanohardness. The obtained results are in good agreement with the results reported by Yari et al. [49].
Fig. 8. The AFM average roughness and nanohardness measurements of: DGEBA; DGEBA/DG-PDMS; DGEBA/DG-PDMS/MEP-POSS; DGEBA/DG-PDMS/OEP-POSS.
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The increase in hardness can be correlated with the storage modulus data: the samples become more rigid as POSS with higher functionalities was used. The reinforcing effect of the POSS was thus confirmed and a good interfacial adhesion between polymeric matrix and POSS compounds was gained in the tested composites. The contact angle measurements were also performed and no significant differences were observed between the synthesized samples. When POSS was added, a slight increase of the surface hydrophobicity was observed probably to the hydrophobic nature of -Si-O-Si- linkage. In the case of MEP-POSS based nanocomposites, the tendency to form agglomerations is supplementary confirmed also by the contact angle measurements. 4. Conclusions To prepare DGEBA/DG-PDMS/POSS nanocomposites with good dispersion and improved thermo-mechanical properties, a low amount of epoxy functionalized polydimethylsiloxane (DG-PDMS) together with POSS bearing one or eight epoxy groups (10 wt.%) were covalently introduced within DGEBA epoxy matrix through copolymerization of epoxy groups. The presence of DG-PDMS as a spacer between POSS molecules and DGEBA leads to a nanoscale dispersion of OEP-POSS within the polymer matrix. Thus, for this nanocomposite the highest thermo-mechanical properties were obtained. According to DSC data, the introduction of (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS) within DGEBA/DG-PDMS matrix, leads to an increase of the temperature at which the polymerization enthalpy of epoxy groups is maximum (Tmax) and depends mainly on the number of the epoxy groups belonging to the POSS cages. The inclusion of OEP-POSS within DGEBA/DGPDMS matrix, leads to a significantly increase of the maximum polymerization enthalpy of the epoxy groups, the exothermic peak being shifted from 143 C to 157 C. In this case the polymerization reaction occurs with a lower rate. The DSC data are in good agreement with the FTIR kinetic studies which demonstrate that the inclusion of both DG-PDMS and POSS compounds reduces the polymerization rate curing process of DGEBA/DG-PDMS/POSS nanocomposites, but after the postcuring treatment, all the studied specimens exhibit complete cure. The DMA results show that the presence of DG-PDMS reduces the glass transition temperature indicating that DG-PDMS forms a homogeneous phase with the epoxy resin. Moreover, the average roughness obtained from the AFM measurements evidenced that DGEBA/DG-PDMS hybrid and DGEBA homopolymer exhibit similar surface topographies and similar roughness values indicating that DG-PDMS is well dispersed within DGEBA epoxy matrix and no phase segregation occurs. Similar results were obtained from the SEM and Si mapping images. The incorporation of POSS functionalized with one or eight epoxy groups (10 wt.%) within DGEBA/DG-PDMS networks exhibits a significantly influence on the morphology and thermomechanical properties (Tg, storage modulus, thermal stability and hardness) of the obtained nanocomposites. Among POSS based nanocomposites enhanced thermomechanical properties were obtained for those reinforced with OEP-POSS. These features could be ascribed to the nanoscale reinforcement effect of OEP-POSS on the DGEBA/DG-PDMS matrix. This statement is supplementary sustained by the SEM/EDX and AFM micrographs that emphasize the higher compatibility between OEP-POSS and the selected polymer matrix, this type of POSS being well dispersed in the polymer matrix due to the eight epoxy groups attached to POSS molecules.
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Acknowledgment The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/ 159/1.5/S/132397.
[25]
[26]
References
[27]
[1] Laine RM, Choi J, Lee I. Organiceinorganic nanocomposites with completely defined interfacial interactions. Adv Mater 2001;13(11):800e3. [2] Gnanasekaran D, Madhavan K, Reddy BSR. Developments of polyhedral oligomeric silsesquioxanes (POSS), POSS nanocomposites and their applications: a review. J Sci Ind Res 2009;68(6):437e64. [3] Phillips SH, Haddad TS, Tomczak SJ. Developments in nanoscience: polyhedral oligomeric silsesquioxane (POSS)-polymers. Curr Opin Solid State Mater Sci 2004;8(1):21e9. jka L, Amici Kroutilov [4] Mate a I, Lichtenhan JD, Haddad TS. Structure ordering and reinforcement in POSS containing hybrids. Eur Polym J 2014;52:117e26. [5] Kannan RY, Salacinski HJ, Butler PE, Seifalian AM. Polyhedral oligomeric silsesquioxane nanocomposites: the next generation material for biomedical applications. Acc Chem Res 2005;38(1):879e84. [6] Zhang W, Müller AHE. Architecture, self-assembly and properties of welldefined hybrid polymers based on polyhedral oligomeric silsequioxane (POSS). Prog Polym Sci 2013;38(8):1121e62. [7] Kuo SW, Chang FC. POSS related polymer nanocomposites. Prog Polym Sci 2011;36(12):1649e96. [8] Nagendiran S, Alagar M, Hamerton I. Octasilsesquioxane-reinforced DGEBA and TGDDM epoxy nanocomposites: characterization of thermal, dielectric and morphological properties. Acta Mater 2010;58(9):3345e56. [9] Potter WG. Epoxide resins. New York: Springer; 1970. [10] Chen XM, Ellis B. Coatings and other applications of epoxy resins. In: Bryan E, editor. Chemistry and technology of epoxy resins. London: Chapman & Hall; 1993. p. 303e25. Chapter 9. [11] Ornaghi Jr HL, Pistor V, Zattera AJ. Effect of the epoxycyclohexyl polyhedral oligomeric silsesquioxane content on the dynamic fragility of an epoxy resin. J Non-Cryst Solids 2012;358(2):427e32. [12] Lu T, Liang G, Peng Y, Chen T. Blended hybrids based on silsesquioxaneeOH and epoxy resins. J Appl Polym Sci 2007;106(6):4117e23. jka L, Murias P, Plestil J. Effect of POSS on thermomechanical properties [13] Mate of epoxyePOSS nanocomposites. Eur Polym J 2012;48(2):260e74. [14] Fina A, Monticelli O, Camino G. POSS-based hybrids by melt/reactive blending. J Mater Chem 2010;20:9297e305. [15] Chrusciel JJ, Lesniak E. Modification of epoxy resins with functional silanes, polysiloxanes, silsesquioxanes, silica and silicates. Prog Polym Sci. http://dx. doi.org/10.1016/j.progpolymsci.2014.08.001. [16] Murias P, Maciejewski H, Galina H. Epoxy resins modified with reactive low molecular weight siloxanes. Eur Polym J 2012;48(4):769e73. [17] Canto CF, A Prado LAS, Radovanovic E, Yoshida IVP. Organiceinorganic hybrid materials derived from epoxy resin and polysiloxanes: synthesis and characterization. Polym Eng Sci 2008;48(1):141e8. jka L. Crosslinking of [18] Gonzalez M, Kadlec P, Stepanek P, Strachota A, Mate epoxy-polysiloxane system by reactive blending. Polymer 2004;45(16): 5533e41. [19] Meng Y, Wei Z, Liu L, Liu L, Zhang L, Nishi T, et al. Significantly improving the thermal stability and dispersion morphology of polyhedral oligomeric silsesquioxane/polysiloxane composites by in-situ grafting reaction. Polymer 2013;54(12):3055e64. [20] Zheng Y, Tan Y, Dai L, Lv Z, Zhang X, Xie Z. Synthesis, characterization, and thermal properties of new polysiloxanes containing 1,3-bis(silyl)-2,4-dimethyl2,4-diphenylcyclodisilazane. Polym Degrad Stab 2012;97(11):2449e59. [21] Chen D, Nie J, Yi S, Wu W, Zhong Y, Liao J, Huang C. Thermal behaviour and mechanical properties of novel RTV silicone rubbers using divinyl-hexa[(trimethoxysilyl)ethyl]-POSS as cross-linker. Polym Degrad Stab 2010;95(4): 618e26. [22] Lungu A, Florea NM, Iovu H. Dimethacrylic/epoxy interpenetrating polymer networks including octafunctional POSS. Polymer 2012;53(2):300e7. [23] Sulca NM, Lungu A, Garea SA, Iovu H. Monitoring the synthesis of new polymer nanocomposites based on different polyhedral oligomeric silsesquioxanes using Raman spectroscopy. J Raman Spectrosc 2009;40(11): 1634e40. [24] Lungu A, Sulca NM, Vasile E, Badea N, Parvu C, Iovu H. The influence of POSS substituent on synthesis and properties of hybrid materials based on urethane
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35] [36]
[37] [38] [39] [40]
[41]
[42] [43]
[44]
[45] [46]
[47]
[48]
[49]
dimethacrylate (UDMA) and various polyhedral oligomeric silsesquioxane (POSS). J Appl Polym Sci 2011;121(5):2919e26. Constantin F, Garea SA, Voicu G, Iovu H. The influence of the organic substituents from the polyhedral oligomeric silsesquioxane (POSS) cage on the epoxy-poss hybrid materials properties. High Perform Polym 2010;22(8): 905e15. Constantin F, Garea SA, Sandu T, Iovu H. Characterization of novel epoxy composites based on hybrid Inorganic/organic polyhedral oligomeric silsesquioxane. Int J Polym Anal Ch 2010;15(2):119e28. Sulca NM, Lungu A, Voicu G, Garea SA, Iovu H. Advanced characterization of polyhedral oligomeric silsesquioxanes used for nanocomposites synthesis. Mat Plast 2009;46(2):124e8. Sulca NM, Lungu A, Popescu R, Garea SA, Iovu H. New polymeric nanocomposites based on polyhedral oligomeric silsesquioxanes. Mat Plast 2009;46(1):1e10. Garea SA, Constantin F, Voicu G, Iovu H. New nanocomposites based on epoxy resin and modified montmorillonite with polyhedral oligomeric silsesquioxane-amine compounds. Mat Plast 2008;45(4):414e20. Constantin F, Garea SA, Iovu H. The influence of organic substituents of polyhedral oligomeric silsesquioxane on the properties of epoxy-based nanomaterials. Compos Part B 2013;44(1):558e64. Balanuca B, Lungu A, Hanganu AM, Stan LR, Vasile E, Iovu H. Hybrid nanocomposites based on POSS and networks of methacrylated camelina oil and various PEG derivatives. Eur J Lipid Sci Technol. http://dx.doi.org/10.1002/ejlt. 201300370. Zhang QG, Fan BC, Liu QL, Zhu AM, Shi FF. A novel poly(dimethyl siloxane)/ poly(oligosilsesquioxanes) composite membrane for pervaporation desulfurization. J Membr Sci 2011;366(1e2):335e41. Vibrational and corrosion Rodosek M, Rauter A, Perse LS, Kek DM, Vuk AS. properties of poly(dimethylsiloxane)-based protective coatings for AA 2024 modified with nanosized polyhedral oligomeric silsesquioxane. Corros Sci 2014;85:193e203. Baumann TF, Jones TV, Wilson T, Saab AP, Maxwell RS. Synthesis and characterization of novel PDMS nanocomposites using POSS derivatives as crossLinking filler. J Polym Sci Part A Polym Chem 2009;47(10):2589e96. Paul DR, Mark JE. Fillers for polysiloxane (“silicone”) elastomers. Prog Polym Sci 2010;35(7):893e901. Ionescu C, Craciun LS, Manea MM, Burducea I, Straticiuc M, Racolta PM, et al. Measurements of the mechanical properties and structure of a DLC film before and after 19F irradiation. J Optoelectron Adv M 2012;14(11e12):971e5. Jee AY, Lee M. Comparative analysis on the nanoindentation of polymers using atomic force microscopy. Polym Test 2010;29(1):95e9. Diez-Pascual AM, Gomez-Fatou MA, Ania F, Flores A. Nanoindentation in polymer nanocomposites. Prog Mater Sci 2015;67:1e94. Zhang Z, Liang G, Ren P, Wang J. Curing behavior of epoxy/POSS/DDS hybrid systems. Polym Compos 2008;29(1):77e83. Thomas R, Durix S, Sinturel C, Omonov T, Gossens S, Groeninckx G, et al. Cure kinetics, morphology and miscibility of modified DGEBA-based epoxy resin e effects of a liquid rubber inclusion. Polymer 2007;48(6):1695e710. Soares BG, Dahmouche K, Lima VD, Silva AA, Caplan SPC, Barcia FL. Characterization of nanostructured epoxy networks modified with isocyanateterminated liquid polybutadiene. J Colloid Interface Sci 2011;358(2):338e46. Lee SS, Kim SC. Morphology and properties of polydimethylsiloxane-modified epoxy resin. J Appl Polym Sci 1997;64(5):941e55. Song J, Chen G, Wu G, Cai C, Liu P, Li Q. Thermal and dynamic mechanical properties of epoxy resin/poly(urethane-imide)/polyhedral oligomeric silsesquioxane nanocomposites. Polym Adv Technol 2011;22(12):2069e74. Chen WY, Wang YZ, Kuo SW, Huang CF, Tung PH, Chang FC. Thermal and dielectric properties and curing kinetics of nanomaterials formed from possepoxy and meta-phenylenediamine. Polymer 2004;45(20):6897e908. €r E, Yilgo €r I. Silicone containing copolymers: synthesis, properties and Yilgo applications. Prog Polym Sci 2014;39(6):1165e95. Fu J, Shi L, Chen Y, Yuan S, Wu J, Liang X, et al. Epoxy nanocomposites containing mercaptopropyl polyhedral oligomeric silsesquioxane: morphology, thermal properties, and toughening mechanism. J Appl Polym Sci 2008;109(1):340e9. jka L, Mentlik V, Trnka P, Slouf Bo cek J, Mate M. Electrical and thermomechanical properties of epoxy-POSS nanocomposites. Eur Polym J 2011;47(5):861e72. Dodiuk H, Kenig S, Blinsky I, Dotan A, Buchman A. Nanotailoring of epoxy adhesives by polyhedral-oligomeric-sil-sesquioxanes (POSS). Int J Adhes Adhes 2005;25(30):211e8. Yari H, Mohseni M, Messori M, Ranjbar Z. Tribological properties and scratch healing of a typical automotive nano clearcoat modified by a polyhedral oligomeric silsesquioxane compound. Eur Polym J 2014;60:79e91.
Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Novel nanocomposites based on epoxy resin/epoxy-functionalized polydimethylsiloxane reinforced with POSS N.M. Florea a, b, A. Lungu a, P. Badica c, L. Craciun b, M. Enculescu c, D.G. Ghita b, C. Ionescu b, R.G. Zgirian a, H. Iovu a, d, * a
University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu Street, 011061, Bucharest, Romania Horia Hulubei e National Institute for Physics and Nuclear Engineering (IFIN-HH), 30 Reactorului Street, 077125, Magurele, Romania National Institute of Materials Physics, Atomistilor 105 bis, 077125, Magurele, Romania d Academy of Romanian Scientists, 54 Splaiul Independentei, 050094, Bucharest, Romania b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 November 2014 Received in revised form 23 December 2014 Accepted 26 January 2015 Available online 4 February 2015
The purpose of the present study is to develop novel nanocomposites based on diglycidylether of bisphenol A (DGEBA) combined with diglycidylether-terminated polydimethylsiloxane (DG-PDMS), reinforced with 10 wt.% (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS). DG-PDMS and POSS compounds were covalently incorporated into DGEBA resin via copolymerization of epoxy groups. The effect of both DG-PDMS and POSS nanoparticles on the curing reaction, glass transition temperature (Tg), thermal stability, hardness and morphology of DGEBA/DG-PDMS ± POSS nanocomposites were studied by DSC, FTIR, DMA, TGA, SEM/EDX, AFM and contact angle measurements. SEM/EDX and AFM results prove that OEP-POSS is well dispersed within DGEBA/DG-PDMS polymer matrix, while MEP-POSS forms large POSS aggregates. The thermo-mechanical properties of POSS based nanocomposites are also in good correlation with morphology features. MEP-POSS based nanocomposite with heterogeneous dispersion of POSS aggregates exhibits lower Tg value and thermal stability in comparison with OEP-POSS nanocomposite which exhibits a nanoscale dispersion of the POSS cages. The obtained Tg of OEP-POSS based nanocomposite increases with 31 C in comparison with the unreinforced matrix. Moreover, this nanocomposite shows the highest storage modulus (E0 ) and hardness. © 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Polymer-matrix composites (PMCs) A. Nano-structures B. Thermomechanical D. Electron microscopy Polyhedral oligomeric silsesquioxanes (POSS)
1. Introduction Over the past decade, advanced researches have been focused on the development of polymer based nanocomposites, reinforced with well-defined nanostructured compounds, which are expected to provide high performance materials, with potential application in a wide range of technological areas [1]. Recently, polyhedral oligomeric silsesquioxanes (POSS) nanostructures have gained considerable attention as they possess a synergistic combination of constituent properties of conventional inorganic materials and organic polymers, resulting in a new generation of hybrid materials with superior properties [2e5]. POSS molecules architecture is based on a well-defined polyhedral cage
* Corresponding author. University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu Street, 011061, Bucharest, Romania. Tel.: þ40 214023922; fax: þ40 213111796. E-mail address: [email protected] (H. Iovu). http://dx.doi.org/10.1016/j.compositesb.2015.01.043 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
(1.5 nm), with general formula (RSiO3/2)n, n varying from 6 to 18. These macromers have an inorganic silica-like core with a precise geometry, surrounded by organic ligands (R) covalently bonded to each Si atoms placed at the vertices of the polyhedral cage. R may be hydrogen or any alkyl, alkylene, aryl, arylene groups, or organofunctional derivatives. Unlike silica, silicones and other conventional inorganic fillers, the POSS compounds exhibit advantages like monodisperse size, low density, and synthetically wellcontrolled functionalities. The functional groups from the organic substituents bring chemical versatility to POSS nanoparticles and make POSS widely compatible with various organic polymers [6e8]. Epoxy resins, the well-known class of thermosetting polymers, have been widely used as adhesives, electronic encapsulating compounds, and organic phase of composite materials due to their high mechanical strength, high chemical and corrosion resistance, excellent electrical insulation and simplicity in processing [9,10]. For further improvements of the general properties of epoxy resins,
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different POSS compounds containing epoxy, hydroxyl and amino reactive functionalities which can generate covalent bonding with the base polymer to reduce the agglomeration effect, have been employed. In general, the obtained results showed that the incorporation of POSS nanounits within epoxy resins improves the performances of the obtained networks [11e13]. However, even for POSS containing reactive groups, small POSS aggregates can form within the base polymer that lead to lower thermo-mechanical properties [14]. Moreover their highly cross-linked structures exhibit a brittle behavior, poor crack resistance, and low fracture toughness. One of the most promising approaches used to modify the features of such brittle polymers consists in adding of flexible polymers like polysiloxanes, used as spacers between POSS and polymer matrix during the preparation of POSS/polymer nanocomposites. This method has been demonstrated to be efficient in order to prepare nanocomposites with good dispersion and enhanced thermo-mechanical properties [15e19]. Polysiloxanes have been extensively used to improve the epoxy resins properties due to their excellent thermal stability [20]. Moreover, these compounds were also used in the preparation of different POSS/ polysiloxane composites. Chen and coworkers demonstrated that the incorporation of POSS nanoparticles into polysiloxane elastomers can improve the thermal stability of the obtained composites [21]. Recently, we have reported the synthesis and characterization of simultaneous interpenetrating polymer networks (IPN) based on dimethacrylic/epoxy resins via in situ polymerization reinforced with octafunctional POSS (with methacrylate or epoxy groups) [22]. In this study, we successfully proved that the degree of POSS dispersion within the polymer matrix is a key factor that is strongly influenced by the organic substituents grafted onto the POSS cage. The obtained results show that the incorporation of OEP-POSS within the used epoxy resin lead to a decrease of thermomechanical properties (Tg and thermal stability) of the obtained nanocomposites. Thus, based on our previous experience [23e31] we intended to design different organic-inorganic hybrid materials with good dispersion and enhanced thermo-mechanical properties by combining diglycidyl ether of bisphenol A (DGEBA) with a small quantity (2 wt.%) of diglycidyl ether terminated poly(dimethylsiloxane), (DG-PDMS). As reinforcing agents, (mono-/octa) epoxy POSS (POSS-(3-Glycidyl) propoxy-Heptaisobutyl substituted (MEP-POSS) and POSS-Octa-(3-glycidyloxypropyl) dimethylsiloxy) (OEP-POSS)) were included into DGEBA/DG-PDMS organic host. To improve the dispersion of POSS during the preparation of POSS/ polymer nanocomposites, DG-PDMS was used as a spacer between POSS molecules and DGEBA, enabling a high mobility of the epoxy chains which may contribute to a better dispersion of POSS units within the polymer matrix. To the best of our knowledge, all previously studies were focused mainly on synthesis of different POSS/epoxy resin and POSS/PDMS nanocomposites, while DGEBA/DG-PDMS nanocomposites reinforced with epoxy POSS compounds remain unexplored [12e16,32e35]. Thus, we focused our research to evaluate the effect of DG-PDMS and epoxy POSS nanostructures on the properties of epoxy network. The curing behavior of DGEBA/DGPDMS reinforced with epoxy POSS was analyzed by DSC and FTIR spectroscopy. The thermal stability and morphology of the obtained nanocomposites are evaluated by DMA, TGA, SEM/EDX, AFM and contact angle measurements. The hardness of the studied materials was also investigated through the AFM nanoindentation tests.
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2. Experimental section 2.1. Materials The studied networks possess in their structures a mixture of diglycidyl ether of bisphenol A (DGEBA), D.E.R.™ 332 (epoxy equivalent weight of 171e175 g/eq) and diglycidyl ether terminated poly(dimethylsiloxane), (DG-PDMS) (epoxy equivalent weight of 490 g/eq and a Mn ~ 800 g/mol). Two types of epoxy POSS (POSS-(3Glycidyl)propoxy-Heptaisobutyl substituted with Mw ¼ 931.63 g/ mol (MEP-POSS) and POSS-Octa(3-glycidyloxypropyl)dimethylsiloxy) with Mw ¼ 931.11 g/mol (OEP-POSS)) were used as reinforcing agents. 1 methyl imidazole (1MeI) was the anionic initiator for the high temperature polymerization reaction of epoxy groups. All reagents, were supplied by Sigma-Aldrich Chemicals, and used without further purification. The chemical structures of the raw materials are further illustrated in Scheme 1. 2.2. Formation of POSS based DGEBA/DG-PDMS nanocomposites As represented in Scheme 1, two types of POSS based DGEBA/ DG-PDMS nanocomposites were synthesized. Firstly, DGEBA/DGPDMS polymer matrix was prepared by mixing the liquid epoxy resin (DGEBA) with the epoxy functionalized polydimethylsiloxane (DG-PDMS) in 98/2 weight ratio. Further, 10 wt.% MEP or OEP POSS was incorporated within DGEBA/DG-PDMS organic host. To achieve a good dispersion of both DG-PDMS and POSS nanocages within DGEBA resin, the obtained mixture was sonicated 1 h at room temperature using an ultrasonic processor UP100H. After mixing the components, 5 wt.% 1MeI was added. The resulting blends were degassed to remove the entrapped air during mixing, poured into Teflon molds, cured at 120 C for 2 h, and postcured at 160 C for another 3 h. After cooling, the obtained hybrids were removed from the mold and characterized. DGEBA homopolymer was also prepared in the same conditions. In order to check the efficiency of the network formation and to remove any traces of unreacted compounds, the synthesized samples were Soxhlet extracted with a suitable solvent for 2 h and subsequently were dried at room temperature for minimum 24 h. After extraction, no shape deformation was observed for any specimen. 2.3. Characterization Differential scanning calorimetry (DSC). The calorimetric measurements were performed on a Netzsch DSC 204 F1 Phoenix differential scanning calorimeter under a constant nitrogen flow rate (20 mL/min). All the samples (about 4 mg in weight) were nonisothermally heated from 20 to 350 C and the DSC curves were recorded at a heating rate of 10 C/min. Fourier transform infrared (FTIR) spectra were registered on a Vertex 70 Bruker FTIR spectrometer equipped with an attenuated total reflectance (ATR) accessory in order to determine the chemical structure and also the curing degree of all the studied specimens. All FTIR measurements were performed in the ATR-FTIR cell on Ge crystal, at room temperature. The FTIR spectra were recorded using 32 scans in 600e4000 cm1 wavenumber region. The dynamic mechanical behavior of the samples after postcuring was followed on a Tritec 2000 (Triton Technology) instrument using 10 mm 2 mm 20 mm rectangular specimens in a single cantilever bending mode. Tan d was calculated as a function of the temperature ranging from 130 to 200 C using 5 C/min heating
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Scheme 1. The synthesis route of DGEBA/DG-PDMS ± POSS nanocomposites.
rate at a frequency of 1 Hz. The displacement amplitude was 0.05 mm. The experiment was performed in triplicate. The thermogravimetric analysis (TGA) curves were achieved on a Q500 TA instrument. A sample of about 2.5 mg was placed in a platinum crucible and heated from 20 to 650 C at a heating rate of 10 C/min under a constant nitrogen flow rate (balance flow 10 mL/ min, oven flow 90 mL/min). Three different experiments were run for each sample. Scanning electron microscopy (SEM) and Si mapping of DGEBA/ DG-PDMS ± POSS hybrids were carried out using a Zeiss EVO50 scanning electron microscope equipped with a Carl Zeiss NTS energy-dispersive X-ray detector (EDX). Before analysis, cryogenic fracture surfaces were covered with a thin gold layer. Atomic force microscopy (AFM). Surface topography was examined by using MultiMode NanoScope IIID Controller Atomic Force Microscope (Digital Instruments Veeco Metrology Group, Santa Barbara, CA, USA) working in the so-called tapping mode. Images were achieved using a TESP (0.01e0.025 Ohm-cm Antimony (n) doped Si) tip at 1 Hz scan rate. All the measurements were performed in ambient environment. Images were flattened using first order fitting to remove any background tilt. All images were achieved at 10 mm, 5 mm and 2.5 mm size scale. In order to characterize the roughness of these samples, a 5 5 mm2 surface area was analyzed. The roughness measurements were performed over the entire image. A minimum of five specimens were tested for each sample and the average values are reported. The hardness of all the studied specimens was also investigated by nanoindentation measurements. Nanoindentation tests were performed at room temperature using the MultiMode NanoScope IIID Controller
atomic force microscope. A DNISP diamond tip mounted on a stainless steel cantilever (spring constant, k ¼ 227 N/m) was used for both indentation test as well as for AFM imaging. Nanoindentation hardness was defined as the maximum indentation load (F) divided by the projected area (A). The projected area was determined from the tip shape of the indenter using AFM NanoScope V531r1 software and Heron formula [36e38]. The indentation load was set to 98 mN and kept constant for all measurements. A 2 2 array of indentation was made on each sample, in three different areas, using the same force and parameters. The hardness was obtained as an average of twelve measurements. Contact angle measurements. The surface wettability was determined using KSV Instruments's CAM 100 by measuring the contact angle on the surface of the films with small water droplets. The contact angles reported represent the mean of three tests. 3. Results and discussion 3.1. The influence of POSS type on the DGEBA/DG-PDMS curing behavior studied by DSC To investigate the effect of both DG-PDMS and POSS type on the reactivity of epoxy groups during copolymerization reactions of DGEBA/DG-PDMS ± POSS nanocomposites, non-isothermal DSC scans were performed. Fig. 1 shows the DSC curves before curing for DGEBA and hybrids based on DGEBA/DG-PDMS ± POSS (MEP or OEP-POSS). The curve 1 from Fig. 1 reveals that DGEBA homopolymer exhibits an exothermic peak at 142 C, assigned to the temperature at
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Fig. 2. FTIR spectra of DGEBA/DG-PDMS system at different polymerization times. Fig. 1. DSC curves for: (1) DGEBA; (2) DGEBA/DG-PDMS; (3) DGEBA/DG-PDMS/MEPPOSS; (4) DGEBA/DG-PDMS/OEP-POSS.
which the polymerization enthalpy of epoxy groups is maximum (Tmax). As one may notice from Fig. 1 (curve 2), the presence of DGPDMS exhibits no significant influence on the polymerization of epoxy groups from pure DGEBA. The introduction of (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS) within DGEBA/DGPDMS matrix, causes an increase of Tmax which depends mainly on the number of the epoxy groups belonging to the POSS cages. Thus, the inclusion of MEP-POSS within DGEBA/DG-PDMS matrix, leads to a slightly increase of Tmax from 143 C to 147 C (curve 3). This type of POSS bearing only one reactive group exhibits a high agglomeration tendency which may lead to a partial POSS cages agglomeration which hinders the attack of the initiator (1-MeI) to the epoxy groups. When OEP-POSS is used, the epoxy polymerization occurs at a higher temperature (157 C, Fig. 1 curve 4) probably due to the restricted mobility of the DGEBA chains in the vicinity of the POSS cages, these chains being less available for the polymerization. The reactivity of epoxy groups belonging to this type of POSS is much lower than for neat DGEBA. In the case of OEP-POSS/1MeI the exothermic peak assigned to the maximum polymerization enthalpy of the epoxy groups appears at 194 C, as we previously reported [22].
ðA =A Þ h ¼ 1 915 1608 t 100 ðA915 =A1608 Þ0
(1)
where:
h e conversion, %;
A915 ¼ 915 cm1 peak area; A1608 ¼ 1608 cm1 peak area; t ¼ the reaction time (min); 0 ¼ initial moment of the reaction (uncured mixture). The obtained conversion degrees for DGEBA/DG-PDMS ± POSS nanocomposites are shown in Fig. 3. From the conversion results, it may be observed that the conversion degree of epoxy groups is strongly influenced by the presence of both DG-PDMS and POSS compounds (Fig. 3). From Fig. 3 curve 2, one may noticed that the incorporation of a small amount of low molecular weight DG-PDMS into DGEBA epoxy resin leads to lower conversions than for neat DGEBA probably due to the chains of DG-PDMS which may partially hinder the attack of 1MeI molecules to the epoxy groups.
3.2. FTIR studies during the polymerization of POSS based DGEBA/ DG-PDMS nanocomposites FTIR studies were used to monitor the curing reactions of the epoxy groups. The FTIR spectra shown in Fig. 2, were performed during the curing process of DGEBA/DG-PDMS hybrid and reveal that the peak assigned to the epoxy groups (915 cm1) significantly decreases at higher reaction times. After the postcuring treatment the corresponding absorption band disappeared, indicating that the curing reaction of epoxy groups is complete. Under the same curing and postcuring conditions similar results were obtained for POSS-containing nanocomposites. The characteristic peak in the infrared region for the epoxy ring disappeared, indicating that the curing reactions in the nanocomposites were carried out to completion. The band at 1608 cm1, assigned to aromatic C]C stretching vibration was used as a reference. The conversion degree of epoxy groups was calculated by means of Equation (1) from the ratio of the epoxy groups absorbance peak area (915 cm1) relative to that of the aromatic C]C stretching vibration (1608 cm1), for the cured and uncured specimens.
Fig. 3. The conversion degree of the epoxy groups against time for: (1) DGEBA; (2) DGEBA/DG-PDMS; (3) DGEBA/DG-PDMS/MEP-POSS; (4) DGEBA/DG-PDMS/OEP-POSS.
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The inclusion of POSS compounds, bearing one or eight epoxy moieties, decreases the rate of epoxy polymerization especially at the early stages, which is probably a consequence of the hindrance effect due to the bulky POSS cages [22e24,39]. Moreover, a significant influence of the number of epoxy groups from the POSS cages against conversion was noticed. Thus, in the early stages of the reaction this influence is not noticeable since the network formation is just at the beginning. However, when the reaction goes on, the crosslinking density becomes higher since more epoxy groups are involved. As a consequence, OEP-POSS based nanocomposites exhibit a high crosslinking density of the final network due to the high content of epoxy groups from the POSS molecules. In this case the access of the 1MeI molecules to the epoxy groups becomes more difficult which leads to a lower conversion of systems with OEP-POSS in comparison with those with MEP-POSS in which the formed network exhibit a lower crosslinking density. This behavior is probably caused by the agglomeration tendency of MEP-POSS. More details about morphology will be further described in morphology section. To achieve a complete cure for all the studied specimens, a postcuring treatment at a higher temperature was also performed. At the end of the postcuring process, all the systems were almost completely cured (Fig. 3). 3.3. Thermo-mechanical properties To establish the influence of DG-PDMS and POSS on the Tg and storage modulus (E0 ) of the cured DGEBA/DG-PDMS ± POSS networks, DMA tests were performed in the temperature range 130 ÷ 200 C (Fig. 4, Table 1). From the tan d curves it may be observed that the neat epoxy network (Fig. 4(a) curve 1) shows two transitions: one at 137 C assigned to the a-relaxation and corresponds to the glass transition of the epoxy network. The transition at low temperature (130 C to 20 C) is attributed to the b-relaxation of the epoxy chain, and is due to the motion of the glycerol groups in DGEBA [40,41]. The incorporation of 2 wt.% of low molecular weight DG-PDMS within DGEBA leads to a lower Tg of the cured DGEBA/DG-PDMS network (127 C) in comparison with DGEBA homopolymer. This behavior is caused mainly by the high flexibility of DG-PDMS chain. It is also clearly seen that DGEBA/DG-PDMS hybrid displayed a single well-defined Tg, which indicates that the epoxy
Fig. 4. The dependences of tan d (a) and storage modulus E0 (b) curves against temperature for DGEBA/DG-PDMS ± POSS nanocomposites.
Table 1 Thermomechanical results for DGEBA/DG-PDMS ± POSS hybrid materials. DGEBA/DG-PDMS ± POSS
DMA
TGA
Tga, ( C) Td5%b, ( C) Td50%c, ( C) Weight lossd, (%) DGEBA DGEBA/DG-PDMS DGEBA/DG-PDMS/MEP-POSS DGEBA/DG-PDMS/OEP-POSS a b c d
137 127 132 158
383 400 318 335
438 440 440 445
87 88 83 80
Tg ¼ glass transition temperature (maximum of tan d); Td5% loss ¼ the temperature at which the weight loss is 5%. Td50% loss ¼ the temperature at which the weight loss is 50%. The percentage of the weight loss at 600 C.
terminated PDMS form a homogeneous phase with the epoxy resin [16,42,43]. The dynamic mechanical behavior of the obtained PDMS/epoxy hybrids was determined using the storage modulus (E0 ) results. From Fig. 4(b) curve 2 it may be observed that the inclusion of the flexible DG-PDMS chains within DGEBA resin, leads to a lower E0 of the obtained hybrid network, in comparison with E0 of DGEBA neat matrix. For these types of hybrid networks a lower crosslinking density was achieved which lead to a lower E0 in the rubbery region [31]. The incorporation of MEP-POSS within the polymer matrix, leads to a slightly increase of Tg of the obtained nanocomposites in comparison with the unreinforced DGEBA/DG-PDMS organic host. This type of POSS bearing only one reactive group exhibits a selfeassembly tendency which may lead to a partial POSS cages agglomeration which subsequently leads to a lower crosslinking density. When OEP-POSS is used, the obtained value for the Tg of the corresponding nanocomposite is shifted with 31 C towards higher temperature compared with the polymer matrix. The effect of raising the Tg by the introduction of POSS cages into the organic host suggests that the polymeric matrix can be reinforced on the nanoscale level by the POSS cages which are covalently bonded to the polymer networks [8,44]. OEP-POSS seems to be the best candidate to form a highly crosslinked network due to the presence of multiple epoxy groups. The rigid and bulky structure of POSS cages could restrict the motions of macromolecular chains which additionally enhance the Tg of the OEP-POSS based nanocomposites. Moreover, for OEP-POSS based nanocomposites, it was noted that the tan d curve was significantly decreased in height and becomes broader. This suggests higher limitations on freedom of chain mobility in the samples probably due to an increased crosslinking density [31,43]. Concerning the dynamic mechanical properties of the POSS reinforced systems, it was interesting to note that in the glass state region the storage modulus of MEP-POSS reinforced nanocomposites is lower than DGEBA/DG-PDMS. This behavior is probably caused by the heterogeneous structure of the obtained nanocomposites in which the POSS compounds form large aggregates. Moreover, the presence of bulky POSS aggregates leads to an increase in the free volume between the polymer chains thus acting like a plasticizer. In the rubbery domain, the dynamic storage modulus E0 appears to be improved, no matter what type of POSS was used. The highest storage modulus was achieved for the nanocomposites containing OEP-POSS which can easily react with epoxy groups from both DGEBA and DG-PDMS resulting nanocomposites with higher crosslinking density. These networks exhibit a higher stiffness even in comparison with DGEBA pure matrix.
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Fig. 5. TGA plots for: (1) DGEBA; (2) DGEBA/DG-PDMS; (3) DGEBA/DG-PDMS/MEPPOSS; (4) DGEBA/DG-PDMS/OEP-POSS.
3.4. Thermal stability The thermal stability of the DGEBA/DG-PDMS ± POSS hybrid materials was revealed through TGA measurements (Fig. 5). Within the experimental temperature range all the TGA curves are unimodal, displaying only one degradation step, suggesting that the incorporation of DG-PDMS and epoxy POSS exhibits no significantly influence on the thermodegradation mechanism of the epoxy polymer matrix. From the TGA curves, it can be calculated that the Td5% value obtained for DGEBA/DG-PDMS hybrid is 17 C higher compared with the Td5% registered for DGEBA homopolymer. This behavior clearly indicates that the incorporation of DG-PDMS within epoxy resin increases the thermostability of the obtained DGEBA/DGPDMS hybrid. The presence of SieOeSi inorganic backbone on
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the DG-PDMS structure contributes to the increase of the thermostability due to the high SieO bond dissociation energy (460 kJ/ mol) compared with the CeO (345 kJ/mol) or even with SieC (318 kJ/mol) bonds [45]. The nanocomposites reinforced with POSS (10 wt.%) were also subject to thermal analysis. The TGA results show that the MEPPOSS/OEP-POSS based nanocomposites exhibit lower initial thermal stability compared with the unreinforced network, DGEBA/DGPDMS. Among POSS based nanocomposites, the lowest temperature corresponding to the weight loss of 5% was obtained in the case of MEP-POSS nanocomposite which is probably due to the low crosslinking density of the network established between MEP-POSS and epoxy matrix. Although the initial thermal stability is significantly decreased, at higher temperatures, the degradation degree (Td50%) is almost similar for POSS based nanocomposites as for the unreinforced material. As Fu et al. previously reported [46], this behavior can be attributed to the yields of silicon-rich residues, which make the materials more stable at elevated temperatures. Therefore, the thermal degradation behavior of epoxy resins is strongly influenced by the presence of POSS compounds due to the self-decomposition and self-condensation processes which occur at relatively low temperatures and form protective layers that increase the thermostability at higher temperatures. It should be emphasized that the lowest degradation degree at higher temperatures was observed for the nanocomposite reinforced with OEP-POSS, among all the POSS based nanocomposites. This improvement could be the result of the formation of tightly crosslinked structure between the polymer matrix and POSS cages. In this case, the POSS cages are probably dispersed at nanoscale level in epoxy matrix. For the nanocomposite with MEP-POSS, the degradation rate at higher temperatures (Td50%) remains unchanged in comparison with unreinforced matrix. This type of POSS exhibits a strong tendency toward aggregation and thus its presence tends to reduce the epoxy crosslinking density. Therefore, the decomposition
Fig. 6. SEM and EDX Si mapping micrographs of the fractured surface of POSS based DGEBA/DG-PDMS nanocomposites.
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temperature of the epoxy resin will decrease. These results are in good agreement with DMA tests. As we expected, the char yield increases for the POSS based nanocomposites, because of the presence of the inorganic moiety.
3.5. Morphological characterization To establish the morphology of the DGEBA/DG-PDMS ± POSS hybrid materials, SEM with EDX Si map distribution and AFM analyses were performed and the obtained results are displayed in Figs. 6 and 7(aed), respectively.
The SEM and Si mapping images, show that DG-PDMS is homogenously distributed in the corresponding epoxy matrix. SEM and EDX Si mapping micrographs are also performed for the POSS based nanocomposites. The obtained results reveal that the OEPPOSS seems to be well dispersed in the network, and no aggregates were observed. This good dispersion of OEP-POSS hybrid nanobjects can be attributed to the miscibility of the POSS within the resin mass and to the covalent bonds formed between the eight epoxy functionalities belonging to POSS nanocages and the epoxy groups from the base matrix. Studying the morphology of MEP-POSS containing networks, it is obviously that this type of POSS exhibits a highly agglomeration
Fig. 7. The AFM micrographs of (a) DGEBA; (b) DGEBA/DG-PDMS; (c) DGEBA/DG-PDMS/MEP-POSS; (d) DGEBA/DG-PDMS/OEP-POSS.
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tendency that leads to a phase segregation in the matrix which is in good agreement with DMA results. The obtained results are also in good agreement with those reported by Bo cek et al. [47]. To fully understand the morphology of the POSS based materials, AFM analyses were performed on all the synthesized nanocomposites and the obtained 3D images are displayed in Fig. 7(aed). AFM is also a useful tool for studying the surface roughness at the nanoscale level. To characterize the roughness of the studied samples, 5 mm2 surface areas were analyzed and the obtained average roughness (Ra) values are illustrated in Fig. 8. Applying special investigation modes, AFM can additionally be used to measure mechanical properties such as nanoindentation hardness. The results of hardness measurements performed for the studied samples are illustrated also in Fig. 8 along with the roughness values. From the 3D AFM images shown in Fig. 7aeb and from the average roughness (Fig. 8), it may be observed that DGEBA/DGPDMS hybrid and DGEBA homopolymer exhibit similar surface topographies and similar roughness values indicating that DGPDMS is well dispersed within DGEBA epoxy matrix and no phase segregation occurs. For the POSS based nanocomposites different morphologies were obtained (Fig. 7ced). The incorporation of MEP-POSS into DGEBA/DG-PDMS matrix yields to a considerable increase in the surface roughness as shown in Fig. 7c and Fig. 8. MEP-POSS being functionalized with one epoxy group and seven unreactive isobutyl groups, exhibits a high tendency to self-agglomerate at the surface of the obtained nanocomposite. Our results for MEP-POSS are completely consistent with the results obtained by Dodiuk et al. [48] who describes that an increased roughness is accompanied with a reduced Tg and storage modulus. In this case, POSS nanocages act more like a plasticizer than as reinforcing agent. Consecutively, it was concluded, that a poor compatibility is obtained between this type of POSS and the polymer matrix. The nanocomposite with OEP-POSS exhibits a roughness value similar with the unreinforced samples, which means that this POSS is well dispersed in the polymer matrix due to the eight epoxy groups attached to SieOeSi cages. In this case a higher compatibility between the POSS molecules and the polymer matrix was achieved. Concerning the nanoindentation hardness features, it was noticed that the presence of 2 wt.% of the polysiloxane monomer generate minor modifications [16]. As expected, the addition of 10 wt.% POSS into neat DGEBA/DG-PDMS matrix has a great effect on the nanohardness. The obtained results are in good agreement with the results reported by Yari et al. [49].
Fig. 8. The AFM average roughness and nanohardness measurements of: DGEBA; DGEBA/DG-PDMS; DGEBA/DG-PDMS/MEP-POSS; DGEBA/DG-PDMS/OEP-POSS.
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The increase in hardness can be correlated with the storage modulus data: the samples become more rigid as POSS with higher functionalities was used. The reinforcing effect of the POSS was thus confirmed and a good interfacial adhesion between polymeric matrix and POSS compounds was gained in the tested composites. The contact angle measurements were also performed and no significant differences were observed between the synthesized samples. When POSS was added, a slight increase of the surface hydrophobicity was observed probably to the hydrophobic nature of -Si-O-Si- linkage. In the case of MEP-POSS based nanocomposites, the tendency to form agglomerations is supplementary confirmed also by the contact angle measurements. 4. Conclusions To prepare DGEBA/DG-PDMS/POSS nanocomposites with good dispersion and improved thermo-mechanical properties, a low amount of epoxy functionalized polydimethylsiloxane (DG-PDMS) together with POSS bearing one or eight epoxy groups (10 wt.%) were covalently introduced within DGEBA epoxy matrix through copolymerization of epoxy groups. The presence of DG-PDMS as a spacer between POSS molecules and DGEBA leads to a nanoscale dispersion of OEP-POSS within the polymer matrix. Thus, for this nanocomposite the highest thermo-mechanical properties were obtained. According to DSC data, the introduction of (mono-/octa) epoxy POSS nanocages (MEP or OEP-POSS) within DGEBA/DG-PDMS matrix, leads to an increase of the temperature at which the polymerization enthalpy of epoxy groups is maximum (Tmax) and depends mainly on the number of the epoxy groups belonging to the POSS cages. The inclusion of OEP-POSS within DGEBA/DGPDMS matrix, leads to a significantly increase of the maximum polymerization enthalpy of the epoxy groups, the exothermic peak being shifted from 143 C to 157 C. In this case the polymerization reaction occurs with a lower rate. The DSC data are in good agreement with the FTIR kinetic studies which demonstrate that the inclusion of both DG-PDMS and POSS compounds reduces the polymerization rate curing process of DGEBA/DG-PDMS/POSS nanocomposites, but after the postcuring treatment, all the studied specimens exhibit complete cure. The DMA results show that the presence of DG-PDMS reduces the glass transition temperature indicating that DG-PDMS forms a homogeneous phase with the epoxy resin. Moreover, the average roughness obtained from the AFM measurements evidenced that DGEBA/DG-PDMS hybrid and DGEBA homopolymer exhibit similar surface topographies and similar roughness values indicating that DG-PDMS is well dispersed within DGEBA epoxy matrix and no phase segregation occurs. Similar results were obtained from the SEM and Si mapping images. The incorporation of POSS functionalized with one or eight epoxy groups (10 wt.%) within DGEBA/DG-PDMS networks exhibits a significantly influence on the morphology and thermomechanical properties (Tg, storage modulus, thermal stability and hardness) of the obtained nanocomposites. Among POSS based nanocomposites enhanced thermomechanical properties were obtained for those reinforced with OEP-POSS. These features could be ascribed to the nanoscale reinforcement effect of OEP-POSS on the DGEBA/DG-PDMS matrix. This statement is supplementary sustained by the SEM/EDX and AFM micrographs that emphasize the higher compatibility between OEP-POSS and the selected polymer matrix, this type of POSS being well dispersed in the polymer matrix due to the eight epoxy groups attached to POSS molecules.
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Acknowledgment The work has been funded by the Sectoral Operational Programme Human Resources Development 2007-2013 of the Ministry of European Funds through the Financial Agreement POSDRU/ 159/1.5/S/132397.
[25]
[26]
References
[27]
[1] Laine RM, Choi J, Lee I. Organiceinorganic nanocomposites with completely defined interfacial interactions. Adv Mater 2001;13(11):800e3. [2] Gnanasekaran D, Madhavan K, Reddy BSR. Developments of polyhedral oligomeric silsesquioxanes (POSS), POSS nanocomposites and their applications: a review. J Sci Ind Res 2009;68(6):437e64. [3] Phillips SH, Haddad TS, Tomczak SJ. Developments in nanoscience: polyhedral oligomeric silsesquioxane (POSS)-polymers. Curr Opin Solid State Mater Sci 2004;8(1):21e9. jka L, Amici Kroutilov [4] Mate a I, Lichtenhan JD, Haddad TS. Structure ordering and reinforcement in POSS containing hybrids. Eur Polym J 2014;52:117e26. [5] Kannan RY, Salacinski HJ, Butler PE, Seifalian AM. Polyhedral oligomeric silsesquioxane nanocomposites: the next generation material for biomedical applications. Acc Chem Res 2005;38(1):879e84. [6] Zhang W, Müller AHE. Architecture, self-assembly and properties of welldefined hybrid polymers based on polyhedral oligomeric silsequioxane (POSS). Prog Polym Sci 2013;38(8):1121e62. [7] Kuo SW, Chang FC. POSS related polymer nanocomposites. Prog Polym Sci 2011;36(12):1649e96. [8] Nagendiran S, Alagar M, Hamerton I. Octasilsesquioxane-reinforced DGEBA and TGDDM epoxy nanocomposites: characterization of thermal, dielectric and morphological properties. Acta Mater 2010;58(9):3345e56. [9] Potter WG. Epoxide resins. New York: Springer; 1970. [10] Chen XM, Ellis B. Coatings and other applications of epoxy resins. In: Bryan E, editor. Chemistry and technology of epoxy resins. London: Chapman & Hall; 1993. p. 303e25. Chapter 9. [11] Ornaghi Jr HL, Pistor V, Zattera AJ. Effect of the epoxycyclohexyl polyhedral oligomeric silsesquioxane content on the dynamic fragility of an epoxy resin. J Non-Cryst Solids 2012;358(2):427e32. [12] Lu T, Liang G, Peng Y, Chen T. Blended hybrids based on silsesquioxaneeOH and epoxy resins. J Appl Polym Sci 2007;106(6):4117e23. jka L, Murias P, Plestil J. Effect of POSS on thermomechanical properties [13] Mate of epoxyePOSS nanocomposites. Eur Polym J 2012;48(2):260e74. [14] Fina A, Monticelli O, Camino G. POSS-based hybrids by melt/reactive blending. J Mater Chem 2010;20:9297e305. [15] Chrusciel JJ, Lesniak E. Modification of epoxy resins with functional silanes, polysiloxanes, silsesquioxanes, silica and silicates. Prog Polym Sci. http://dx. doi.org/10.1016/j.progpolymsci.2014.08.001. [16] Murias P, Maciejewski H, Galina H. Epoxy resins modified with reactive low molecular weight siloxanes. Eur Polym J 2012;48(4):769e73. [17] Canto CF, A Prado LAS, Radovanovic E, Yoshida IVP. Organiceinorganic hybrid materials derived from epoxy resin and polysiloxanes: synthesis and characterization. Polym Eng Sci 2008;48(1):141e8. jka L. Crosslinking of [18] Gonzalez M, Kadlec P, Stepanek P, Strachota A, Mate epoxy-polysiloxane system by reactive blending. Polymer 2004;45(16): 5533e41. [19] Meng Y, Wei Z, Liu L, Liu L, Zhang L, Nishi T, et al. Significantly improving the thermal stability and dispersion morphology of polyhedral oligomeric silsesquioxane/polysiloxane composites by in-situ grafting reaction. Polymer 2013;54(12):3055e64. [20] Zheng Y, Tan Y, Dai L, Lv Z, Zhang X, Xie Z. Synthesis, characterization, and thermal properties of new polysiloxanes containing 1,3-bis(silyl)-2,4-dimethyl2,4-diphenylcyclodisilazane. Polym Degrad Stab 2012;97(11):2449e59. [21] Chen D, Nie J, Yi S, Wu W, Zhong Y, Liao J, Huang C. Thermal behaviour and mechanical properties of novel RTV silicone rubbers using divinyl-hexa[(trimethoxysilyl)ethyl]-POSS as cross-linker. Polym Degrad Stab 2010;95(4): 618e26. [22] Lungu A, Florea NM, Iovu H. Dimethacrylic/epoxy interpenetrating polymer networks including octafunctional POSS. Polymer 2012;53(2):300e7. [23] Sulca NM, Lungu A, Garea SA, Iovu H. Monitoring the synthesis of new polymer nanocomposites based on different polyhedral oligomeric silsesquioxanes using Raman spectroscopy. J Raman Spectrosc 2009;40(11): 1634e40. [24] Lungu A, Sulca NM, Vasile E, Badea N, Parvu C, Iovu H. The influence of POSS substituent on synthesis and properties of hybrid materials based on urethane
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35] [36]
[37] [38] [39] [40]
[41]
[42] [43]
[44]
[45] [46]
[47]
[48]
[49]
dimethacrylate (UDMA) and various polyhedral oligomeric silsesquioxane (POSS). J Appl Polym Sci 2011;121(5):2919e26. Constantin F, Garea SA, Voicu G, Iovu H. The influence of the organic substituents from the polyhedral oligomeric silsesquioxane (POSS) cage on the epoxy-poss hybrid materials properties. High Perform Polym 2010;22(8): 905e15. Constantin F, Garea SA, Sandu T, Iovu H. Characterization of novel epoxy composites based on hybrid Inorganic/organic polyhedral oligomeric silsesquioxane. Int J Polym Anal Ch 2010;15(2):119e28. Sulca NM, Lungu A, Voicu G, Garea SA, Iovu H. Advanced characterization of polyhedral oligomeric silsesquioxanes used for nanocomposites synthesis. Mat Plast 2009;46(2):124e8. Sulca NM, Lungu A, Popescu R, Garea SA, Iovu H. New polymeric nanocomposites based on polyhedral oligomeric silsesquioxanes. Mat Plast 2009;46(1):1e10. Garea SA, Constantin F, Voicu G, Iovu H. New nanocomposites based on epoxy resin and modified montmorillonite with polyhedral oligomeric silsesquioxane-amine compounds. Mat Plast 2008;45(4):414e20. Constantin F, Garea SA, Iovu H. The influence of organic substituents of polyhedral oligomeric silsesquioxane on the properties of epoxy-based nanomaterials. Compos Part B 2013;44(1):558e64. Balanuca B, Lungu A, Hanganu AM, Stan LR, Vasile E, Iovu H. Hybrid nanocomposites based on POSS and networks of methacrylated camelina oil and various PEG derivatives. Eur J Lipid Sci Technol. http://dx.doi.org/10.1002/ejlt. 201300370. Zhang QG, Fan BC, Liu QL, Zhu AM, Shi FF. A novel poly(dimethyl siloxane)/ poly(oligosilsesquioxanes) composite membrane for pervaporation desulfurization. J Membr Sci 2011;366(1e2):335e41. Vibrational and corrosion Rodosek M, Rauter A, Perse LS, Kek DM, Vuk AS. properties of poly(dimethylsiloxane)-based protective coatings for AA 2024 modified with nanosized polyhedral oligomeric silsesquioxane. Corros Sci 2014;85:193e203. Baumann TF, Jones TV, Wilson T, Saab AP, Maxwell RS. Synthesis and characterization of novel PDMS nanocomposites using POSS derivatives as crossLinking filler. J Polym Sci Part A Polym Chem 2009;47(10):2589e96. Paul DR, Mark JE. Fillers for polysiloxane (“silicone”) elastomers. Prog Polym Sci 2010;35(7):893e901. Ionescu C, Craciun LS, Manea MM, Burducea I, Straticiuc M, Racolta PM, et al. Measurements of the mechanical properties and structure of a DLC film before and after 19F irradiation. J Optoelectron Adv M 2012;14(11e12):971e5. Jee AY, Lee M. Comparative analysis on the nanoindentation of polymers using atomic force microscopy. Polym Test 2010;29(1):95e9. Diez-Pascual AM, Gomez-Fatou MA, Ania F, Flores A. Nanoindentation in polymer nanocomposites. Prog Mater Sci 2015;67:1e94. Zhang Z, Liang G, Ren P, Wang J. Curing behavior of epoxy/POSS/DDS hybrid systems. Polym Compos 2008;29(1):77e83. Thomas R, Durix S, Sinturel C, Omonov T, Gossens S, Groeninckx G, et al. Cure kinetics, morphology and miscibility of modified DGEBA-based epoxy resin e effects of a liquid rubber inclusion. Polymer 2007;48(6):1695e710. Soares BG, Dahmouche K, Lima VD, Silva AA, Caplan SPC, Barcia FL. Characterization of nanostructured epoxy networks modified with isocyanateterminated liquid polybutadiene. J Colloid Interface Sci 2011;358(2):338e46. Lee SS, Kim SC. Morphology and properties of polydimethylsiloxane-modified epoxy resin. J Appl Polym Sci 1997;64(5):941e55. Song J, Chen G, Wu G, Cai C, Liu P, Li Q. Thermal and dynamic mechanical properties of epoxy resin/poly(urethane-imide)/polyhedral oligomeric silsesquioxane nanocomposites. Polym Adv Technol 2011;22(12):2069e74. Chen WY, Wang YZ, Kuo SW, Huang CF, Tung PH, Chang FC. Thermal and dielectric properties and curing kinetics of nanomaterials formed from possepoxy and meta-phenylenediamine. Polymer 2004;45(20):6897e908. €r E, Yilgo €r I. Silicone containing copolymers: synthesis, properties and Yilgo applications. Prog Polym Sci 2014;39(6):1165e95. Fu J, Shi L, Chen Y, Yuan S, Wu J, Liang X, et al. Epoxy nanocomposites containing mercaptopropyl polyhedral oligomeric silsesquioxane: morphology, thermal properties, and toughening mechanism. J Appl Polym Sci 2008;109(1):340e9. jka L, Mentlik V, Trnka P, Slouf Bo cek J, Mate M. Electrical and thermomechanical properties of epoxy-POSS nanocomposites. Eur Polym J 2011;47(5):861e72. Dodiuk H, Kenig S, Blinsky I, Dotan A, Buchman A. Nanotailoring of epoxy adhesives by polyhedral-oligomeric-sil-sesquioxanes (POSS). Int J Adhes Adhes 2005;25(30):211e8. Yari H, Mohseni M, Messori M, Ranjbar Z. Tribological properties and scratch healing of a typical automotive nano clearcoat modified by a polyhedral oligomeric silsesquioxane compound. Eur Polym J 2014;60:79e91.